Sadashi,
As I understand, you use the original controller of the inverter airco. You have placed the temperature sensor from the indoor coil on yhe brazed plate heat exchanger, and the air temp sensor on the water return.
But what did you do with the fan of the indoor unit? Did the original controller not complaint when you run the ac without an indoor fan connected?
Love to hear more details on this setup,
Murdock

Sensor position
 

Hi Murdock and welcome to the discussion . There are two sensors with this C&H ( Gree made ) unit that I'm using , one was attached to the copper heat exchanger ( evaporator ) and the other one is the air sensor . The one that was attached to the evaporator I've placed it on the refrigerant liquid line right after is coming out of the brazzed plate heat exchanger and the air temperature sensor I've extended the wire and I've placed it somewhere inside the house on a wall along with the small led display that's got the ir receiver as well incorporated . You have to keep all the electronics of the indoor unit and also the fan motor to be able to control the outdoor unit . You detach the fan motor from the fan and place it somewhere in a box and secure it somehow so it won't move when it starts spinning . I haven't tried to trick the pcb and not keep the motor because I believe the motor got a sensor on the inside that communicates with the pcb and if it detects that the motor does less than 450 rpm it will shut down the unit .

If you investigate the motor a bit more you'll probably find one of the wires is providing a square wave pulse output that varies depending on the speed of the motor.

It should be fairly simple to build a circuit (i.e. 555 timer) to simulate this output and allowing you to dispense with your 'motor in a box'.

Agree with Acuario. It can be relative easily hacked. On the orher hand, I guesse the motor spinning without any load, will probably comsume ony a few watts. All in all, Sadashi, it looks quite easy to modify the airco to an air-water heat pump. Have you got an idea about your COP? Since you haven't altered anything of the outdoor unit, I would assume you 'll have a COP close to factory spec.

I see a few fundamental misconceptions with your project. Just going to ask a few questions to clear up the mystery.

So the system you have running is a constant output unit that you have running full blast for a short time to satisfy a whole day's demand? And you plan to substitute this unit for a variable speed unit of similar capacity? You may pick up a fair amount of efficiency, but at what cost? A finance payment on a new outdoor unit will probably equal the existing unit's electric bill.

If your existing unit has capillary tube metering, you may be able to increase its efficiency at a pretty low cost. There are a few ways to accomplish this. Which way to go depends a lot on the existing unit, as well as your skill set and confidence level.

I'm assuming this system is your primary home heating. Of so, and you don't have sufficient backup heat, a diy solution might not be the best idea. These things tend to fail during the most extreme conditions when you need them the most.

I need to change my system based on two factors.
1. Old unit is not running well under 5degC.
2. Old setup is heating just the water tank, once at the highest temp of the day. House heating is a small wood stove, if I'm not at home is expensive to heat the house with electric radiators.

You are right, the system will fail when you needed most.
My backup remain the wood stove, even if the system will fail the house is very well insulated, loosing 1-2 degree C in one day, depending of outside temperature and sun.

I chose to buy one of the shelf heat pump, but until I save money fot this I can made a cheeper plan.
For the cheeper plan can anybody recommend a system type with high COP? And made it like Sadashi did.

It's more than just high COP in my opinion . You can have high COP at A7/W35 but the unit will loose heating capacity dramatically when the cold weather is coming and you need it most . I would choose a Japanese manufacturer but not Daikin ( it loses about 40-50 % heating capacity at negative temperatures ) and not entry level units . In my opinion is more important to have heating capacity at low temperatures with lower COP , than high COP at positive temperatures and no heating capacity at low temperatures , you need to find a balance between them two and you need to find out how many kW of heating does your house need when you have -15°C , -20°C outside and then look for a unit that can provide those kW of heating at those temperatures . Good luck

If you are looking for a unit to do only slab heating, that makes the system less complicated than one that both heats and cools. If you go with a brazed heat exchanger and a floor slab, you should not need a big water tank. Especially if you are interested in a variable speed compressor unit. That's the big value in using one: no big tank. With a single speed outdoor unit that can dig into the cold temperatures without losing too much capacity, a heat store tank makes a lot more sense.

For domestic hot water heating, there are a few ways to go. You don't need a big heat pump. A few ecorenovators have made heat pump water heaters out of dehumidifiers. Randen made one out of a 5kbtu air conditioner.

Never heard back about your existing unit. If it's got capillary tube metering, you can go with an expansion valve and get better cold climate performance. I just picked up two of them on eBay for about 20 dollars each shipped. What valve to use depends on your unit. Mine will be going into a 2 ton whole house window/wall ac unit that got its face melted in a kitchen accident.